The State Department said Wednesday allegations of a Libyan plot against Saudi Arabia's de facto ruler, Crown Prince Abdullah, have slowed the growth of U.S.-Libyan relations. Saudi Arabia has withdrawn its ambassador from Libya in connection with the plot charges, which Libya denies.

Libya's decision a year ago to renounce weapons of mass destruction has been described as one of the Bush administration's major foreign policy achievements. But officials here acknowledge that progress toward full relations with the Tripoli government has been stalled by the assassination issue.

Details of the alleged conspiracy emerged earlier this year when an Arab-American activist admitted in a U.S. court that he had acted as a go-between for Libyan officials and Saudi dissidents who were to kill the Crown Prince.

Libyan leader Moammar Gadhafi allegedly ordered the killing after he and the de facto Saudi ruler exchanged insults at an Arab League summit in 2003.

Libya has insisted there never was an assassination plot. But at a news briefing here State Department Spokesman Richard Boucher said the United States takes the issue seriously, and that Libya's explanations to date are "not sufficient" for American officials to make a judgment on the matter.

Mr. Boucher said there is no question that U.S.-Libyan relations have advanced since its decision to scrap weapons of mass destruction. But that the issue of the alleged plot has "impacted" the speed of normalization and will continue to do so until it is cleared up.

"We've made an awful lot of progress, and indeed we're doing things with Libya that we weren't doing a year or two ago," he noted. "Libya's decision to get rid of its weapons of mass destruction is major step forward for Libya's role in the international community and for our ability to do things with them. But there are many aspects to this growing relationship and some of them are certainly hampered by these reports and the fact we haven't gotten to the bottom of them yet."

The warming trend in bilateral relations began last year when Libya took responsibility and agreed to pay compensation for the 1988 bombing of a U.S. jetliner over Scotland that killed 270 people.

The United States has sent diplomats to Libya for the first time in more than two decades and has lifted a number of business restrictions against the North African state.

But full normalization is blocked by Libya's continued presence on the State Department's list of state sponsors of terrorism, which is related in part to questions stemming from the alleged conspiracy in Saudi Arabia.

In response to U.S. inquiries on the matter, Libya has told the United States that it does not support the use of violence to settle political differences between states.

Mr. Boucher said the issue came up as recently as two weeks ago in talks between a senior U.S. diplomat and Libyan officials in Rome, and said the United States continues to make clear to the Libyans that it expects them to abide by the stated commitment.

The spokesman said Libya has taken "significant steps" to repudiate past support for terrorism. But officials say that to remove Libya from the terrorism list, the Bush administration would have to certify to Congress that it has not been involved in any such activity for a sustained period, something it cannot do in the absence of a full resolution of the Saudi issue.

Mr. Boucher's remarks came after Saudi Arabia said Wednesday it had withdrawn its ambassador from Tripoli and would expel the Libyan envoy in Riyadh over what it said was Libya's "atrocious" role in the alleged plot. Saudi Arabia, however, said it was not breaking relations with Libya.